11. Andrew Mason

Seizing on the power of collective buying via daily deals on the Internet may seem obvious now. But it’s from Mason’s brain that the multibillion-dollar idea sprang. The Northwestern grad, who cofounded Groupon four years ago with mentor and former employer Eric Lefkofsky (No. 10) and Brad Keywell (No. 21), comes across as naive and even goofy (Google “Andrew Mason doing yoga in his underpants”). Who cares—so far he’s successfully running the hottest Chicago startup in years.